You might want to check out Ruboto. I haven't used it yet, but it seems to allow you to do exactly what you want, i.e. write full Android apps using JRuby. I've heard there's a significant performance penalty, though.
I've also heard of people having success using Mirah to write Android apps. The syntax is very similar to Ruby, so it will be comfortable for you. It compiles to a straight Java class with no runtime dependencies, so you don't get the performance hit you run into with Ruboto.
WTF? In the Mirah examples, I see a bunch of
i = 0
while i < 10
...code...
i += 1
end
Check out ruboto-core at http://ruboto.org/
As an aside...does anyone have experience with PhoneGap or Appcelerator? Wondering about experiences with those approaches (both positive and negative).